Search for Water on Other Planets Takes Giant Leap Forward

A reddish jet of gas emanates from the forming star HH-30, which is surrounded by a protoplanetary disk.

Credit: NASA/ESA

The search for water on other planetary bodies has taken a
giant leap forward in recent months. In November, NASA announced that it had
found substantial quantities of water on the Moon. Earlier this month, the
Cassini spacecraft obtained data about one of Saturn's moons, Enceladus, that may
confirm the presence of sub-surface liquid water.

While these missions scour our solar system for traces
of water  a necessary condition for life  a group of scientists is
looking beyond, at solar systems light years away. A recent study published in
the journal Astrobiology described using infrared spectroscopy to model the
dust surrounding young extrasolar stars to try to detect the presence of hydrous
minerals called phyllosilicates.

One of the simplest examples of phyllosilicates is clay
minerals. Water is an important part of their chemical structure.

"If you find phyllosilicates, you have most likely
found liquid water," says lead author Melissa Morris, a visiting professor
in the Department of Physics, Astronomy and Materials Science at Missouri State
University and an affiliate of Arizona State University's School of Earth and
Space Exploration. "The objective was to try to determine whether we could
actually detect these wonderful signatures of hydrated minerals almost always
produced by the interaction of liquid water with rock."

In order to determine whether the surface of an extrasolar
planet would contain water, scientists can look at what is called the protoplanetary
disk  a disk of gas and dust surrounding a star during its early stages of
development. Scientists think planets are born from protoplanetary disks
through gravitational and electrostatic interactions between particles. So if
scientists can determine the elemental composition of the dusty disks that
orbit young stars, they should be able to predict what sort of planets will eventually
form.

One school of thought suggests that the Earth acquired
its surface water from asteroids or asteroid-like bodies that were present in its
protoplanetary disk. The authors of this study used the same assumption for
potential Earth-like planets in other
solar systems. Therefore, if phyllosilicates are found in the protoplanetary
disk of a young extrasolar star, the assumption is that water would most likely
be found on the surface of planets that are later born within the disk. ?(Of
course, as Mercury, Venus and Mars illustrate, other conditions will affect
whether a rocky planet ultimately has water.)

The scientists hope to someday use instruments such as
the Spitzer Space Telescope and the Stratospheric Observatory for Infrared
Astronomy (SOFIA) to determine the composition of exozodiacal dust in
extrasolar protoplanetary disks. Before that can be done, however, scientists
must first determine if detection of particular minerals in these distant systems
is even possible. This study helps scientists determine what signatures to look
for in disks.

The composition of the dust is identified by studying its
emission features. A common procedure is to use infrared spectroscopy to
identify substances by the infrared wavelengths they absorb or emit. This
procedure is often used to detect water on planetary bodies.

Morris and her colleagues began by modeling the infrared
emission features of dust that did not contain hydrated minerals, or
phyllosilicates. They then changed the mineral mixture by adding
phyllosilicates amounting to three percent of the total mixture.

In the paper, Morris and her co-author Steve Desch of
Arizona State University claim that unique features indicative of phyllosilicates
in the mid-infrared spectra should make it possible to detect those minerals in
protoplanetary disks.

Scott Sandford, a research astrophysicist at the NASA Ames
Research Center in California who has experience conducting spectroscopy in
meteorites, disagrees. He says proving the presence of phyllosilicates in a
protoplanetary disk is a challenge.

"It is somewhat difficult to identify phyllosilicates
when they are present in mixtures because they are relatively featureless as
opposed to other minerals, which have a lot of structural features in their
spectrum," says Sandford.

Morris says the outcome of this study shows only that, based
on the computer models, it should be possible to detect the presence of
phyllosilicates in protoplanetary disks. It is only the first step in the detection
of water in other solar systems.

"My part was developing the model to determine whether
it could be done," says Morris. "What instruments are available? Of the
instruments we have, do they have the resolution?"

The next step, which Morris has already begun, is to apply
this technique to actual data. Morris is now comparing the models to data
obtained from the Spitzer Space Telescope.

Sandford says that will be the real test.

"The basic idea they are espousing is a perfectly good
one," says Sandford. "I'm personally kind of skeptical that you can
locate the phyllosilicates in this disk to the level they suggest. How applicable
are those models to the real world?"

Morris says this type of research is also important in
understanding how planetary systems form in general.

"I'm a huge advocate for looking for water in our own
solar system," says Morris, "but just to understand the process of
planetary system formation, we need to go outside
our solar system and look at other systems as well."